When you go into A Great Fairy Fountain, you will be able to upgrade
your current weapon. The upgrades are as follows

Bomb: Makes the bomb bigger.
Boomerang: Allows you to throw the boomerang across the screen at
a further distance.
Bow: Allows you to shoot 3 arrows at a time.
Fire Rod: Allows you to shoot flames in 4 directions.
Lamp: No upgrade.
Magic Hammer: Allows you to emit larger impact radius.
Pegasus Boots: Allows you to run over holes and pits.
Roc's Feather: Allows you to perform a double jump.
Shovel: Allows you to detect treasure or secret passages with
sound.
Slingshot: Allows you to shoot 3 stones at a time.

Obtain the Fire Rod and use it on a group of bushes. All of the
bushes will reveal one Force Gem each.
Information in this section was contributed by MelloProto.
To get 300 Force Gems, obtain the shovel in the 2nd area of level
2 and dig behind the house with the 2 pots on the top and many
pots inside of it (southeast) to uncover a big Purple Force Gem that
is worth 300.

To get 100 Force Gems, in Level 2 there are several areas with
little green extrusions on the ground. These are Big Green Force
Gems worth 100. To get them, obtain a shovel and dig them up. There
are also Big Purple Force Gems worth 300.

Obtain the Fire Rod, Bombs, or Lamp and start a group of bushes,
grass, tree stumps, etc. on fire. For each individual tile of grass
and bushes, you will get one Force Gem. Set a stump on fire to
receive a small Blue Force Gem worth 5.

Obtain the Pegasus Boots and run into trees and other objects to
uncover hidden Force Gems.

In Area 1 Level 3, get a level 2 shovel. In the area with Tingle
trying to take a very large Force Gem, there will be hole by a tree.
Find it, go in, and when you come out, go to the Force Gem. It will
say "It's good to be alive!".

There is a way to get across horizontal gaps. The only downside is
that you must be going to the right. Go to the edge of the platform
then hold C-Right. When you get to the edge of the platform, get on
quickly and release C-Right then press the Analog-stick Right. ***
If you press the Analog-stick Right before you get to the platform,
you will fall.

To round up the thieves in Level 5 Area 2, stun them with your sword
then pick them up with R. Bring them to the screen with a house and
4 treasure chests. Throw them into the fence on the top right of
the screen.

In some areas in level 2, there will be places that flash and have a
certain sound when you are near them. This indicates that you need a
Moon Pearl to go into the "secret world" where all of the children
are located.

When playing the Tower Of Winds level in Shadow Battle, go to the
Dark World. It contains 4 switches. If you are able to hit all
four, a Giant Bomb will fall into the Light World, destroying
anything that remains. If you are able to, grab another player who
is still in Light World and keep him there to really make them mad.

Complete the Hyrulean Adventure to unlock the last five
Shadow Battle arenas.
Hint: Mini-games at Tingle's Tower:
Tingle's Tower is accessed by bringing 2 or more players to a
already completed level, or by completing the 2nd stage of a
level with 2 or more players.

When playing the Tower Of Winds level in Shadow Battle, go to the Dark World. It contains
four switches. If you are able to hit all four, a Giant Bomb will fall into the Light
World, destroying anything that remains. If you are able to, grab another player who is
still in Light World and keep him there to really make them mad.